This specification claims the priority of Japanese Patent Application No. 6-333329, filed on Dec. 15th, 1994.
1. Field of the Invention
The present invention relates to a converter for converting image data in NTSC format into that in PAL format, and, more particularly, to a memory configuration for an apparatus applied to images obtained by an electronic endoscope.
2. Description of the Prior Art
A TV monitor used for an electronic endoscope or the like generally employs the NTSC (National Television System Committee) system. When images obtained by the electronic endoscope are intended to be displayed on a PAL (Phase Alternation by Line) system monitor, an NTSC-PAL converter is useful. That is, the number of scanning lines is 525 lines in the NTSC system and 625 lines in the PAL system. Scanning line data for 525 lines is converted into scanning line data for 625 lines by interpolation.
FIG. 5 shows the process of NTSC-PAL conversion for frame data, in which data D1 of an odd-numbered line NO1 in the NTSC format stored in each field memory is added to data D2 of an even-numbered line NE1, data of (D1+D2)/2 is read as data for an odd-numbered line PO1 in PAL format, and data for odd-numbered line PE1 is read according to data of an even-numbered line NE1 and that of an odd-numbered line NO2. Similarly,
format line data P02, PE2, . . . are sequentially read. As shown in the figure, for example, two line data PO3 and PE3 consisting of duplicated data (same data) are read for the PAL system based on line data NO3 and NE3 in the NTSC format, and two line data P06 and PE6 consisting of duplicated data are similarly read based on line data NE5 and NO6.
Thus, data of PAL system with 625 scanning lines can be obtained from data of NTSC system with 525 scanning lines by reading and interpolating the same data in duplicate. With the above converter, image signals in the NTSC format from an electronic endoscope and other various devices can be easily converted into those in the PAL format, thereby being able to reduce the cost and the number of control steps with a common design.
However, the above-mentioned conventional NTSC-PAL converter has a problem such that bar noise occurs in the horizontal direction because the time for creating image data of one field or frame differs for each system. FIG. 6 shows the relationship between image data in NTSC format written in a field memory and frame data in PAL format read from that memory on a transversal time base. That is, the converter is provided with two field memories (1) and (2) for storing images in the NTSC format. Odd-numbered field data 0 (ODD) N0, ON1, ON2, . . . are written in the field memory (1), and even-numbered field data E (EVEN) N0, EN1, EN2, . . . are written in the field memory (2), these data being alternately written while being updated. The writing is performed every field cycle of 16.6 ms (1/60 seconds).
Then, under the read control shown in FIG. 5, image data in the PAL format, that is, odd-numbered frame data OP1, even-numbered frame data EP1, odd-numbered frame data OP2, even-numbered frame data EP2, . . . are formed every 20 ms (1/50 seconds). The odd-numbered frame data OP1 is formed basing on the field data ON1 and EN1, as shown in the figure, while the even-numbered frame data EP1 is formed basing on the field data EN1 and ON2.
However, as described above, the time for writing one field data in the field memories (1) and (2) is for the NTSC system, and 1/60 seconds=16.66 . . . ms, while the read time for image data of the PAL system is 1/50 seconds=20 ms. This time difference causes the above writing to overtake the reading. That is, FIG. 7 shows the data write/read status by noticing an overtake point B in FIG. 6. At the overtake point B, the line NO4 of the odd-numbered field data ON3 from the field memory (1) is added to the line NE4 of the even-numbered field data EN3 from the field memory (2), and the line PE4 for the frame data OP3 is read.
However, at the point B, the data for line NO4 in the field memory (1) is updated, and the writing overtakes the reading at the end of the line NO4 so that, on and after the line NO5, the line PO5 for the frame data OP3 would be generated based on the rewritten data. Therefore, in the frame data OP3, there occurs a break (shifting of time base) at the point B. The break in data may not cause a serious problem in a still image, but generates a horizontal bar noise in a dynamic image, particularly, in a one with large amount of motion.
The present invention is made in view of the above problems, and intended to provide an NTSC-PAL converter which can eliminate generation of bar noise, and can convert image data in the NTSC format into that in the PAL format with less memory capacity.